Fix disk_usage when `df' has long output …
disk_usage relies on the column based awk to parse the result of df. It
works fine in most cases but failed to handle the special cases when df
has long output like this:

    $ df -l
    Filesystem           1K-blocks      Used Available Use% Mounted on
    /dev/mapper/VolGroup00-LogVol00                      49898164   2187920  45134680   5% /
    /dev/sda1               101086     13039     82828  14% /boot
    tmpfs                   190224         0    190224   0% /dev/shm

New method aligns at Use%, and slice all the characters before that in
each line.